1. Field of Invention
The invention relates to a capacitive transducer of the type comprising a metallic transducer-housing having two electrically conducting plates mounted thereon or therein, one of which constitutes a stationary electrode and the other one of which constitutes an electrode which is movable relative to the stationary one. The movable electrode is mounted at the end of the transducer housing, while the stationary electrode is mounted on an insulating body secured in the interior of the transducer housing and there supports the stationary electrode at a small distance from the said movable electrode.
2. Description of the Prior Art
A transducer of the above mentioned kind may be, for example a condenser microphone. The invention is of major importance in connection with condenser microphones of a certain quality such as studio microphones or measurement microphones. All measurement microphones are, with very few exceptions, designed as condenser microphones because the design concept for this kind of microphone, more than all other principles, makes it possible to meet the overall requirements which should be met by high-quality measurement microphones. A primary requirement is that the acoustical performance of the microphone is good in order to achieve great accuracy of measurement. It is further necessary that its sensitivity to variations in the environment such as pressure, temperature and humidity is low.
In order to obtain reproducible results and to prolong the intervals between necessary calibrations it is also imperative that the microphone exhibits short-term as well as long-term stability. Further, it should be possible to carry out the calibration in a simple manner, to readily verify its sensitivity and frequency response and to predict its performance not only by means of direct measurements but also by means of calculations based on theoretical considerations which can give an independent confirmation of the data measured.
Condenser microphones for measurement purposes or studio use are commonly made up of mechanical elements which are assembled or joined together by means of threads. These parts or elements form essentially cylindrical structural members which at convenient places are provided with the proper threadings or tappings. The main elements of a condenser microphone are a stationary electrode, also called a backplate, and a movable electrode embodied as a diaphragm which, when at rest, is kept at a well defined distance from the backplate. These two electrodes constitute the parallel plates of a capacitor employing ordinary atmospheric air as the dielectric. The stationary electrode or backplate is screwed to a relatively thick disc of a highly insulating and dimensionally stable material. The disc-shaped insulator is clamped to the inner surface of a tubular microphone-housing of for instance Monel.RTM., titanium or German silver. A stretched foil or diaphragm, which in high-quality transducers is made of metal or metal alloys, is mounted at the end of the microphone housing. This foil or diaphragm constitutes the movable electrode. The microphone housing, insulator and diaphragm form a closed compartment. The occurrence of a pressure difference between the outer atmosphere and the closed compartment causes the diaphragm to be moved or displaced which movement or displacement causes a change of capacity which can be measured electrically. The frequency response of the microphone is determined essentially by the resonance point of the diaphragm and by its damping. The resonance frequency is determined by the mass of the diaphragm and by its mechanical tension. The damping depends on the mobility of the air in the space between the diaphragm and the backplate, and therefore it can be varied partly by choosing an appropriate geometry for the backplate and partly by choosing an appropriate distance between the diaphragm and the backplate.
Because variations in atmospheric pressure vastly exceed the small pressure variations originating in the propagation of sound, at least one pressure equalization vent leading from the closed compartment to the outer atmosphere is provided. The internal diameter of the vent and its length are so adapted that a pressure equalization from the outer atmosphere to the interior cavity of the microphone can take place at slow variations of the atmospheric pressure but prevents pressure equalizations at normally occurring sound frequencies. For the most commonly used types of microphones the lower cut-off frequency of the pressure equalization system ranges from 1 Hz to 10 Hz.
The function of the backplate, in addition to its serving as the stationary electrode of a capacitor, is to influence by its presence close to the diaphragm the movement or displacement of the diaphragm in order to achieve a desired frequency response.
In modern types of microphones, the distance between the electrodes typically ranges from 10 microns to 30 microns. For individual types the choosen distance must be within tolerances typically ranging from 2 to 5 percent, plus/minus, i.e. from 0.2 micron to 1.5 microns, if a suitably uniform damping of the diaphragm displacement in the region about the resonance frequency is to be obtained in practice. In this way the desired uniformity in frequency response and sensitivity of the microphone is obtained. The backplate influences the movement of the diaphragm by dissipating energy as the air in the narrow space between the stationary electrode and the movable electrode is pumped to and fro during the movement of the diaphragm. This damping of the diaphragm movement is usually controlled by the provision of a suitable number of properly sized holes in the backplate which lead from the narrow space between the electrodes to the rear surface of the stationary electrode within the closed compartment of the microphone. For a given type of microphone it is in this way possible to achieve a desired damping factor for the movements of the diaphragm.
In order to make it possible to manufacture microphones which under the most varied environmental conditions operate in a stable manner, i.e. without changing their characteristics, it is of the utmost importance that during the design process care is taken in selecting materials and to ensure that the necessary accuracy of manufacture is established for the individual structural members or bodies.
For long-term stability the materials have to exhibit initial stability. With respect to the insulator a further requirement is made. For measurements at low frequencies the insulator should be made of a highly insulating material implying in practice that ceramics, glass, sapphire, quartz or related materials should be used. Such materials typically have a very low thermal coefficient of linear expansion, a coefficient differing very much from that of metals. This is of importance because the other structural members of the microphone are made of matched metals or their alloys. This may influence the microphones temperature coefficient resulting in sudden changes in the microphones sensitivity during changes in the ambient temperature.
The sensitivity of a condenser microphone is directly proportional to the distance between the electrodes. With the above mentioned figures in mind an inaccuracy in the distance between the electrodes of 0.2 micron results typically in a deviation of 1% from the desired or nominal sensitivity which for certain purposes is unacceptable.
Additonally, the sensitivity of a condenser microphone is inversely proportional to the inner tension of the diaphragm. As this tension is dependent on the extension of the foil it has to be fixed relative to the microphone housing in a well-defined manner.
In the manufacture of high-quality microphones metals are generally used for the diaphragm and the microphone housing. The thermal coefficient of linear expansion of the metals employed ranges from 8.times.19.sup.-6 per degree centigrade to 22.times.10.sup.-6 to per degree centigrade. In good designs materials having a mutual difference in thermal coefficient substantially below 1.times.10.sup.-6 per degree centigrade are selected. This is a necessary measure because the extension of the foil resulting in the desired tension of the membrane only amounts to a few microns. Therefore, an extension of the foil caused by the temperature has to be compensated for by a corresponding expansion of the microphone housing. An important problem of the prior art microphones is that the observance of the necessary tolerances for the distance between the electrodes implies an extensive manufacturing process involving many different time-consuming processes. As examples hereon one may mention plane or surface grinding, machine lapping and simultaneous polishing or finishing of the microphone housing and the bakcplate because those members cannot be manufactured individually with the required tolerances. These processes ensure the parallel relationship between the reference plane of the diaphragm constituted by the diaphragm's abutment surface on the microphone housing and the stationary electrode. Other working processes may be mentioned such as mechanical separation of parts, trimming, buffing and cleaning and subsequently a final assembling which is time-consuming because the correct distance between the electrodes is ensured by the insertion of very thin adjusting washers either between the movable electrode and its abutment surface on the microphone housing or between the insulator disc and its abutment surface on the housing.
Additionally, a further problem occurs in that the insulator material exhibits a thermal coefficient of linear expansion which differs substantially from those of metals. It is therefore necessary to mount the insulating disc in such a way that the microphone housing at the location in which the diaphragm is secured remains uninfluenced by the much lesser expansion of the insulator. In prior art microphones this is achieved by so fitting the insulating disc in the microphone housing that these two members can slide mutually on contiguous surfaces which are perpendicular to the longitudinal axis of the microphone, and the same measure is provided for the mounting of the backplate on the insulating disc. This mounting or assembling procedure results, depending on the practical workmanship, in a risk for discontinuous changes of the sensitivity.